Recently, lithium ion batteries, especially, cylindrical lithium ion batteries, are widely used in various kinds of portable electronic devices, such as video cameras, laptop personal computers, portable DVDs and personal digital assistants, due to high energy density, high working voltage and long life span.
At present, much more attentions have been paid to the safety performance of lithium ion batteries because of the use of high-energy chemical materials and energy concentration. Conventionally, to improve the safety performance of a lithium ion battery, a positive temperature coefficient (PTC) thermistor is provided in the lithium ion battery. When there is a high current in the lithium ion battery because of internal short circuit or external short circuit, the PTC thermistor can remarkably reduce the occurrence of safety accidents.
However, for the batteries which need to discharge at high discharge rate for use in electric tools, for instance electric vehicle batteries, the PTC thermistor cannot be used any more. Consequently, there is still no effective protecting device to improve the safety performance of the lithium ion batteries which need to discharge at high discharge rate. Safety accident possibly occurs in the discharge process of these kinds of lithium ion batteries.
What is needed, therefore, is to provide a cap assembly with desirable safety performance for use in lithium ion batteries.